Mitochondrial Expression and Functional Activity of Breast Cancer Resistance Protein in Different Multiple Drug-Resistant Cell Lines

The multidrug resistance (MDR) phenotype is characterized by the overexpression of a few transport proteins at the plasma membrane level, one of which is the breast cancer resistance protein (BCRP). These proteins are expressed in excretory organs, in the placenta and blood-brain barrier, and are involved in the transport of drugs and endogenous compounds. Because some of these proteins are expressed in the mitochondria, this study was designed to determine whether BCRP is expressed at a mitochondrial level and to investigate its function in various MDR and parental drug–sensitive cell lines. By using Western blot analysis, immunofluorescence confocal and electron microscopy, flow cytometry analysis, and the BCRP (ABCG-2) small interfering RNA, these experiments showed that BCRP is expressed in the mitochondrial cristae, in which it is functionally active. Mitoxantrone accumulation was significantly reduced in mitochondria and in cells that overexpress BCRP, in comparison to parental drug–sensitive cells. The specific inhibitor of BCRP, fumitremorgin c, increased the accumulation of mitoxantrone significantly in comparison with basal conditions in both whole cells and in mitochondria of BCRP-overexpressing cell lines. In conclusion, this study shows that BCRP is overexpressed and functionally active in the mitochondria of MDR-positive cancer cell lines. However, its presence in the mitochondria of parental drug–sensitive cells suggests that BCRP can be involved in the physiology of cancer cells. [Cancer Res 2009;69(18):7235–42]